Security features, e.g. for security documents, can be classified into “covert” security features one the one hand, and “overt” security features on the other hand. The protection provided by covert security features relies on the concept that such features are difficult to detect, typically requiring specialized equipment and knowledge for detection, “overt” security features rely on the concept of being easily detectable with the unaided human senses, e.g. such features may be visible and/or detectable via the tactile sense, while still being difficult to produce and/or to copy. However, the effectiveness of overt security features depends to a great extent on their easy recognition as a security feature, because most users, and particularly those having no prior knowledge of the security features of a therewith secured document or item, will only then actually perform a security check based on said security feature if they have actual knowledge of their existence and nature.
It is known in the art to use inks containing magnetically orientable pigments, particularly also magnetic optically variable pigments, for the production of overt security elements, e.g. in the field of security documents.
WO 2005/002866 A1, for example, discloses improved optically variable printed security elements, comprising a high-resolution image represented by oriented optically variable magnetic pigment particles in a hardened coating layer. Said high-resolution image is produced in a printing process, using a particular magnetic orienting device. The security document is first imprinted with a coating composition comprising magnetic or magnetizable particles, such as optically variable magnetic pigment flakes. The imprinted document is subsequently exposed, while the coating is still “wet”, to the magnetic field of the magnetic orienting device, comprising a magnetized permanent-magnetic plate engraved with indicia. The magnetic or magnetizable particles of the coating are oriented under the influence of the magnetic field of the orienting device, hereby forming an image of said engraved indicia. The coating is subsequently hardened, so as to “freeze” the magnetic or magnetizable particles in their positions and orientations.
Optically variable magnetic pigments which can be used for this purpose have been disclosed for example in U.S. Pat. No. 4,838,648 and in EP 686,675 B1. Corresponding inks and coating compositions have been disclosed in WO 2007/131833 A1.
WO 2008/046702 A1 discloses further types of magnetically induced images produced through the orientation of optically variable magnetic pigment particles in a coating layer, as well as a device for producing said types of images. The device comprises the combination of a magnetized permanent-magnetic plate engraved with indicia with one or more additional magnets mounted with respect to the engraved magnetic plate such as to hold them against the inherent magnetic forces occurring between them.
WO 2004/007095 A2 discloses a device for orienting magnetic pigment flakes in a coating layer exposed to the magnetic field of one or more dipole magnets, to produce aesthetically appealing, bright optically variable coatings, which, albeit flat, show a smooth variation of color and reflectivity with changing viewing angle, which is reminiscent of a floating or moving three-dimensional object. Particularly noteworthy, the device of WO 2004/007095 allows for orienting magnetic pigment flakes in a coating layer so as to produce a “rolling-bar” effect in the resulting coating. A printed “rolling bar” type image shows a contrasting band which appears to move (“roll”) as the image is tilted, and which can be obtained with a single printing step and using a single ink formulation. Printed elements which exhibit an apparent motion of image features with changing viewing angle, such as a “rolling-bar” type effect, are anti-copy protection measures for security documents, which can be easily recognized and used for the authentication of a security document. The device of WO 2004/007095, however, may show the drawback that useful “rolling-bar” type effects can only be produced over a relatively small length, and might thus be often difficult to recognize as a security feature.
This is illustrated in FIGS. 1a and 1b, which each schematically show a dipole magnet DM generating a magnetic dipole field, and a substrate S positioned within said magnetic field on one side of and at a distance d from the magnet DM and essentially parallel to its magnetic axis, i.e., the virtual line between its magnetic north pole and south pole. The dipole magnet in FIG. 1a has a longer extension L along its magnetic axis than the dipole magnet in FIG. 1b along its respective magnetic axis. In both cases, reflective particles, e.g. pigment particles, are provided dispersed within a layer of liquid binder material on top of the substrate S and are oriented by the magnetic field, typically such that the respective longest extension of each particle is substantially aligned with the field lines of the magnetic field at the position of the particle. As can be seen by comparing FIGS. 1a and 1b, the dipole magnet used in the device for orienting the particles in a corresponding layer on a substrate S must have at least the length L over which the “rolling-bar” effect is to be produced. The long dipole magnet DM shown in FIG. 1a has only weakly curved field lines at the location of the substrate S, whereas the short dipole magnet DM shown in FIG. 1b has field lines having a higher degree of curvature at the location of the substrate S. As a consequence, the resulting “rolling-bar” print in case of the long magnet of FIG. 1a shows a large bright zone z, which only exhibits a slight apparent movement with changing viewing-angle, i.e. a poor and hardly eye-catching dynamic effect, whereas in comparison the resulting “rolling-bar” print in case of the short magnet of FIG. 1b shows only a small bright zone z, which however exhibits a strong apparent movement with changing viewing-angle. However, due to the rather limited length corresponding to the short dipole magnet, the security feature is not eye-catching and not easy to recognize, particularly if one does not have any prior knowledge of its presence and/or optical effect. Therefore, there remains a need for security features displaying an eye-catching dynamic optical effect over an extended length.